This invention relates to a method of improving the adhesion of metal applied on the surface of a polyimide.
During the past few decades, printed circuit boards have become the predominant vehicle for mounting and interconnecting electronic components in order to manufacture a desired electronic circuit. The printed circuit board typically consists of a sheet of a dielectric, i.e., nonconductive, substrate constructed of various filled or unfilled synthetic materials such as phenolics, glass-impregnated epoxies, and the like. The substrate is provided with a pattern of thin metal foil which functions as a conductive path on one or both sides. The conductive paths, also referred to as "traces", are usually formed of a conductive material such as a metal, e.g., copper, palladium, nickel, gold, and the like. The traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations on the board.
Plastics such as polyimides and polyphenylene oxides are particularly suitable as materials for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and easy moldability. However, polyimide substrates are not easily provided with a strongly adherent metal trace. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during subsequent manufacturing steps or during use of the circuit board.
Several attempts have been made to increase the adhesion of the conductive metal traces to the polyimide substrates. Adhesion is generally measured as "peel strength", i.e., the force under controlled conditions required to peel the trace from the substrate. For example, chemical and physical surface treatment techniques are often used to physically modify the as-molded polyimide surface and thereby improve the adhesion of metal subsequently applied thereto. The polyimide surfaces can be grit-blasted to provide a roughened surface which anchors the subsequently-applied metals. Furthermore, chemical swelling agents or penetrants can be used to swell the surface, while chemical etching agents can be used to remove portions of the surface.
While such methods do increase adhesion, they are often not entirely satisfactory for several reasons. Such techniques result in degradation of the molecules forming the polyimide surface, and may decrease both the tensile and impact strength of the substrate due to swelling and cracking of the entire substrate material, especially in those instances in which the polyimide contains fillers. Grit blasting requires cumbersome equipment and impedes the processing of printed circuit boards.
The primary object of this invention is to provide a process for improving the adhesion of metal to a polyimide surface without physical modification or degradation of the surface structure.
An additional object of this invention is to provide a process for increasing the adhesion of metallic traces to a polyimide surface while maintaining the ability of the surface to be patterned by various techniques.